vibrochamp blowing power tube

[tweedydum]

does anyone have an idea why my vibrochamp blows power tubes about once every 6 months? I don't use it terribly much, but this has happened twice now. When I has died I have been running it all out, more or less.

I can get some voltages later tonight - just wanted to prime the pump and see what people suggest -

thanks!

Dave

[billyz]

Check the bias current and run better 6v6's. I find most of the new 6v6's to be crap. I have seen tweed champs pulling 40ma on the power tube and run forever with Good old American Tubes. Check your plate voltage though to calculate the wattage. I usually add a 470ohm screen resistor while I am in there, even though it is not technically needed.

Check and or replace your cathode bypass cap too, you can upgrade the voltage rating to 50 or 100vdc. your cathode resistor should be 470 ohm, if it has drifted lower the current will be increased.

[JD0x0]

what tubes are you using?

FYI JJ's dont sound the best but theyre built like tanks (which is the reason they dont sound great)

[Phil_S]

Aside from a short, the fuse is really meant to blow when there is excessive current draw. I think the other guys are on the right track with the discussion about the power tube. Check the components attached to the power tube socket to be sure they are all in-spec, good solders, etc. If you are using a tube rectifier, I'd check that, as well.

If you want a very clear check on the current draw by your power tube, lift the power rail connection on the output transformer and then determine the number of ohms across the primary winding (from the lifted wire to the plate connection on the power tube -- pull the tube when you do this). Then determine the voltage drop across the primary on the output transformer. Do your math: Ohms Law, V=I * R. Solve for I (V/R) and that is a very accurate way of making the determination.

To get the voltage drop, just set your meter for DCV and hook it onto the power tube plate (pin 3) and the spot where the OT is connected to the B+ rail. Use clips for this; it's easier that way.

[jjman]

I put a 750 ohmer on the cathode of mine. Down to an idle of 12watts from 17. Schematic implies about 19watts idle

[JD0x0]

i forgot to add that tubes today are pretty mediocre quality. theres members in another thread that they went through several sets of mullard EL34's and they all went bad bedore they were broken in. And the plate voltages were way below the tubes maximum handling capabilitys.

so it's pretty safe to say you might have gotten two bad sets

[tweedydum]

ok, I will check plate voltages tonight and repost. I have been using electroharmonix 6v6 from wherever they might come from. Thanks for your help.
Dave

[JD0x0]

ok, I will check plate voltages tonight and repost. I have been using electroharmonix 6v6 from wherever they might come from. Thanks for your help.
Dave

Thats your problem right there. electro harmonix tubes are utter garbage, no offence. buy a set of JJ's they'll sound a bit warmer and last a hell of alot longer

[Structo]

Since it is a cathode biased amp, measure your plate voltage and the bias current it is at.

You most likely have the bias too hot and maybe the plate voltage is on the high side as well.